This invention is concerned with the introduction of liquid samples into the source region of a mass spectrometer. The invention has particular applicability as an interface between the output of a liquid chromatograph (LC) and a mass spectrometer, enabling the use of the mass spectrometer as an analyzer for the LC. There has recently been much effort devoted to this problem, a summary of which is provided in the paper entitled, "On-Line Liquid Chromtography Mass Spectrometry: The Monitoring of HPLC Effluents by a Quadrapole Mass Spectrometer and a Direct Liquid Inlet Interface (DLI)", published by Patrick J. Arpino, in the Instrumental Applications in Forensic Drug Chemistry Proc. Int. Symp., May 29-30, 1978, edited by Klein, Kruegel, and Sobol.
Among the methods mentioned in that article for introducing the LC effluent into the mass spectrometer are several in which the liquid leaving the LC is instantly vaporized at or near the tip of a capillary tube; the same is thereby introduced into the mass spectrometer source in gaseous form. Such a system is also described in U.S. Pat. No. 3,997,298 entitled, "Liquid Chromatography--Mass Spectrometry System and Method", issued Dec. 14, 1976, to F. W. McLafferty, et al. It has been found to be a deficiency in such systems that the capillary frequently becomes plugged with residue of the sample and the solvent in which the sample is dissolved; this occurs apparently because of evaporation at the solvent-vapor interface within the capillary tip when the temperature rises above the solvent boiling point. Another prior art system in which the liquid is heated and vaporized is discussed by H. R. Udseth, R. G. Orth, and J. H. Futrell, in a paper entitled, "An LC-MS Interface" presented at the Annual Conference on Mass Spectrometry in St. Louis, Missouri, in June 1978.
A different system is discussed by Tsuge, Hirata, and Takeuchi in an article entitled, "Vacuum Nebulizing Interface for Direct Coupling of Micro-Liquid Chromatograph and Mass Spectrometer", published in Analytical Chemistry, Volume 51, No. 1, January 1979. In this system, a high velocity carrier gas streams by the liquid to form an aerosol, which is then sprayed into a pumping region, where the higher volatility solvent is pumped out. This system may prove to be disadvantageous in that it discriminates in favor of materials of low volatility, more highly volatile materials being preferentially pumped away together with the solvent.